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Define frequency dependent selection, and describe ways it can arise in
nature. Give examples whenever possible.
When we think about the fitness of a particular allele, it is easy to imagine it
as some fixed quantity; a particular trait bestows a particular advantage to an
individual regardless of the context. In nature, however, this is far from the case in
most situations. Just how beneficial a trait might be can vary greatly with many
factors – one interesting context being the frequency of that allele in the rest of the
population. This is known as frequency dependent selection.
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Two distinct types: positive and negative. Both are underpinned by the same
principle but have very different consequences for the allele in question.
Negative FDS arises when an allele is more beneficial when it is rare. This can
prevent the extinction of alleles because as their frequency declines, the
fitness advantage they convey increases, resulting in the propagation of more
copies of the allele into further generations. Thus negative FDS maintains
polymorphisms and increases genetic diversity.
Classic example is apostatic selection. Predators target familiar morphs of a
prey species due to search image formation, and so less common forms will
enjoy less predation. Fitzpatrick et al (2009) presented wild ground foraging
birds with two morphs of model salamanders – a striped form and an unstriped form. In experimental manipulations, the most common form was
disproportionately likely to be attacked by birds, giving an advantage to the
rarer morph.
Similarly, predators that are more unusual may have a selective advantage
due to avoidance image formation by prey. Less well documented.
Rare male effect: interplay with sexual selection. Hughes et al. (2013)
investigated freshwater guppies. Males of this fish species exist in two forms
with distinct tail colour patterns. Manipulating the relative frequencies of
each form, they showed that females consistently chose to mate with the rarer
colour pattern. This may help to reduce inbreeding and thus increase
offspring survival.
Smithson (2001) studied morphs of orchids: one normal, and one deceptive
form that produced no nectar. When rare, the deceptive form did very well,
as it was still visited and pollinated by insects, without producing costly
nectar. However, as this form became more common, insects would avoid the
plants as they learnt they would not be rewarded with nectar and the
deceptive form would become rare once again.
This idea of taking advantage of others can be extended to pathogens like
viruses. When rare, a virus strain will do well since there will be little
immunity. However if it becomes common, the host species will gain
immunity (and reach the critical threshold of herd immunity) so that the
virus can no longer spread. Hence negative FDS drives the evolution of new
viral strains.
Dawkins (1982) discussed a type of particularly selfish gene called meiotic
drive genes or ‘segregation distorters’. These have the effect of altering
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meiosis such that they have a >50% chance of finding themselves in a
successful gamete. These do well when rare because their ‘victims’ are other
alleles – but when common these segregation distorters have a higher chance
of being homozygous which can render the organism virtually sterile.
Positive FDS has quite the opposite of negative. It occurs when the more
common form has the advantage, and so outcompetes the rare forms. Clear
examples are less abundant because the very nature of positive FDS leads to
allele fixation and hence we cannot study it.
Interesting example: the snail Partula sutularis, exists in two forms of shell
coiling (Johnson, 1982). If a dextral coiling snail mates with a sinistral one,
fewer young are produced than if same-coiling snails mate. Therefore in a
population in which one form happens to dominate, positive FDS will favour
the more common morph, since like-for-like matings produce more offspring.
This has produced regional variations in morph distribution, with either
sinistral or dextral forms dominating certain areas.
A more classic example is aposematism – warning colouration in
venomous/poisonous organisms. Aposematism is most effective when it is
the most common phenotype – predators will be more likely to avoid eating
an organism they easily recognise as poisonous. This can spread
interspecifically by Müllerian mimicry – other harmful species will converge
on a similar warning pattern to ensure they too are recognised as toxic and
are not attacked.
Batesian mimicry can manipulate this setup. Harmless species will also avoid
being attacked by converging on the same warning pattern (e.g. the harmless
king snake copying the highly venomous coral snake). However as the
harmless mimic becomes more common predators may learn that the
colouration is not harmful, and so will eat more of animal with that
colouration – some negative FDS again.
Frequency dependent selection involves the interplay of selection with
feedback loops: negative feedback promotes stable polymorphisms in negative
FDS, while positive feedback drives alleles to fixation in positive FDS. Looking at
selection in such specific context is crucial to our comprehending of how the
traits we see in organisms today have evolved. Understanding how fitness varies
with frequency is also going to be vital in areas like conservation; enabling us to
build more complex models to predict and manage humans devastating effect on
biodiversity.